Is using a passphrase secure?

Because words from the passphrase list can be repeated and put in any order, this means there are an exponential number of potential pass-phrases.

Each word in your passphrase can be any word in the 2048 words, so the chance of generating the same passphrase is 1 in 5,444,517,870,735,015,415,413,993,718,908,291,383,296. (~10^39). This post on Reddit should help you visualize the astoundingly small probability of ending up with the same passphrase as someone else.